1. Field of the Invention
The present invention relates to a method of and an apparatus for reading out image information, by scanning an image recording sheet having a stimulable phosphor layer with stimulating light, and detecting photostimulated luminescent light emitted from the sheet with an image detector that has a semiconductor as its primary portion.
2. Description of the Related Art
It is well known in the image reading field to utilize a stimulable phosphor sheet to obtain image information. The stimulable phosphor stores part of radiation energy when exposed to radiation, and exhibits photostimulated luminescence (PSL) according to the stored energy when exposed to stimulating light, such as visible light, etc. The radiation image information of a subject, such as a human body, etc., is temporarily recorded on the stimulable phosphor sheet having a stimulable phosphor layer. The stimulable phosphor sheet emits photostimulated luminescent light when scanned with a stimulating light beam such as a laser light beam. The photostimulated luminescent light is detected photoelectrically and converted to an image signal carrying the radiation image information. As the image information reading apparatus, there have been proposed a wide variety of apparatuses that vary in manner of scanning a stimulating light beam and converting photostimulated luminescent light to an image signal.
For example, as disclosed in Japanese Unexamined Patent Publication Nos. 55(1980)-12492 and 56(1981)-11395, a photomultiplier has been widely employed as a photoelectric converter, which detects photostimulated luminescent light emitted from a stimulable phosphor sheet when scanned with a stimulating light beam. The photomultiplier, however, has the following problems:
1) The photomultiplier is low in shock resistance, large in size, and high in cost, because it is constructed of a hollow glass tube.
2) The photomultiplier is fairly difficult to make thin, as it uses a complex multistage dynode to double photons. The cost of manufacturing a long photomultiplier 17 inches in width would become unduly high.
3) The quantum efficiency of the photocathode utilizing an external photoelectric effect is low. The quantum efficiency with respect to photostimulated luminescent light of wavelength 300 to 500 nm (blue light band) is typically as low as about 10 to 20%, whereas the quantum efficiency with respect to photostimulated luminescent light of wavelength 600 to 700 nm (red light band) is relatively great, typically about 0.1 to 2%. For this reason, a special stimulating-light cut filter becomes necessary to obtain a satisfactory signal-to-noise ratio (S/N ratio) and results in an increase in the manufacturing cost.
Hence, in order to solve the problems associated with the photomultiplier, Japanese Unexamined Patent Publication Nos. 58(1983)-121874, 60(1985)-111568, 60(1985)-236354, xe2x80x9cRadiographic Process Utilizing a Photoconductive Solid-State Imager (772/Research disclosure, Oct. 1992/34264),xe2x80x9d and Japanese Unexamined Patent Publication No. 7(1995)-76800, disclose that a semiconductor sensor (hereinafter referred to as a solid-state image detector) is used instead of the photomultiplier. The solid-state image detector employs a semiconductor (such as ZnS, ZnSe, CdS, TiO2, ZnO, xcex1-SiH, CdS(Cu), ZnS(Al), CdSe, PbO, xcex1-Se, etc.) as a photoconductor.
The solid-state image detectors consisting of the above-mentioned semiconductors, however, have the following problems: photostimulated luminescent light is feeble, so a photoconductor to be used is required to have extremely high dark resistance, but the disclosed photoconductors are all low in dark resistance; and since image reading is performed under a relatively strong electric field, the dark current increases and is superposed on the current component carrying an image signal and develops as dark latent image noise in a reproduced image. Particularly, xcex1-SiH, CdS(Cu), ZnS(Al), CdSe, and PbO, whose bandgap is small, have a large dark current resulting from thermal excitation, so it is extremely difficult to obtain a satisfactory S/N ratio.
xcex1-Se has relatively high dark resistance and can be employed with 1015xcexa9xc2x7cm (at 10V/xcexcm). However, in the case where xcex1-Se is used for detecting photostimulated luminescent light, the dark resistance thereof is insufficient to obtain an image with a satisfactory S/N ratio and is therefore desired to be greater than 1015xcexa9xc2x7cm. Furthermore, the solid-state image detector has not only the problem of dark current but also the problem that, because a pseudo signal occurs because of residual electric charge left unread, an image with a satisfactory S/N ratio cannot be obtained.
The present invention has been made in view of the aforementioned circumstances. Accordingly, it is the primary object of the present invention to provide an image information reading method and an image information reading apparatus that are capable of obtaining an image whose S/N ratio is satisfactory, without being subjected to the influence of dark current or residual electric charge, when reading out image information from an image recording sheet by use of an solid-state image detector.
To achieve the objects mentioned above and in accordance with the present invention, there is provided a first method of obtaining an image signal, comprising the steps of: preparing an image recording sheet having a stimulable phosphor layer which emits photostimulated luminescent light by a quantity corresponding to stored energy when irradiated with stimulating light; scanning the image recording sheet in horizontal and vertical scanning directions with the stimulating light; and obtaining the image signal by detecting the photostimulated luminescent light emitted by the scanning, with a solid-state image detector having a photoconductor; wherein a signal, output from the solid-state image detector when scanning a non-image region other than an image recording region on the image recording sheet with the stimulating light, is detected as a correction signal; and a non-image component, contained in a signal output from the solid-state image detector when scanning the image recording region with the stimulating light, is suppressed with the correction signal.
In the case where a light beam is used as the stimulating light, the aforementioned horizontal scanning direction refers to a direction in which the stimulating light and the scanning speed are relatively high, while the aforementioned vertical scanning direction refers to a direction in which the stimulating light and the scanning speed are relatively low. On the other hand, in the case where long line light is used as the stimulating light, the horizontal scanning direction refers to a direction in which the line light extends, while the vertical scanning direction refers to a direction in which the line light is moved with respect to the image recording sheet.
The expression xe2x80x9cnon-image region other than an image recording region on the image recording sheetxe2x80x9d is intended to mean a region where photostimulated luminescent light carrying image information is not incident on the solid-state image detector even if the stimulating light is scanned on the region. In connection with this meaning, it does not matter whether or not image information has been recorded on the stimulable phosphor sheet. The non-image region may be a non-image region, formed so that photostimulated luminescent light carrying image information is not emitted, such as a region having no stimulable phosphor layer and a region formed so that radiation energy is not stored by intercepting radiation with a lead plate at the time of photographing. Also, it maybe a region on the stimulable phosphor sheet which corresponds to a portion formed so that photostimulated luminescent light is not incident on the detector with a member for intercepting photostimulated luminescent light, regardless of whether or not image information has been actually recorded. Furthermore, it maybe a region outside the stimulable phosphor sheet where no photostimulated luminescent light occurs.
In the aforementioned first method, the aforementioned correction signal is a signal obtained when scanning the vertical leading edge or trailing edge of the non-image region with the stimulating light. Alternatively, the correction signal is a signal obtained when scanning the horizontal leading edge or trailing edge of the non-image region with the stimulating light. Also, it is preferable that each of the vertical leading and trailing edges and horizontal leading and trailing edges of the non-image region be a region on the image recording sheet immediately before or immediately after the image recording region.
In accordance with the present invention, there is provided a second method of obtaining an image signal, comprising the steps of: preparing an image recording sheet having a stimulable phosphor layer which emits photostimulated luminescent light by a quantity corresponding to stored energy when irradiated with stimulating light; scanning the image recording sheet in horizontal and vertical scanning directions with the stimulating light; and obtaining the image signal by detecting the photostimulated luminescent light emitted by the scanning, with a solid-state image detector having a photoconductor; wherein a signal, output from the solid-state image detector when the stimulating light is in an OFF state, is detected as a correction signal; and a non-image component, contained in a signal output from the solid-state image detector when scanning the image recording region with the stimulating light, is suppressed with the correction signal.
The expression xe2x80x9cthe stimulating light is in an OFF statexe2x80x9d is intended to mean the state in which the stimulating light is not emitted from the stimulating-light source by switching off the stimulating-light source directly with an electrical signal, or switching off a power source for the stimulating-light source.
In accordance with the present invention, there is provided a third method of obtaining an image signal, comprising the steps of: preparing an image recording sheet having a stimulable phosphor layer which emits photostimulated luminescent light in a quantity corresponding to stored energy when irradiated with stimulating light; scanning the image recording sheet in horizontal and vertical scanning directions with the stimulating light; and obtaining the image signal by detecting the photostimulated luminescent light emitted by the scanning, with a solid-state image detector having a photoconductor; wherein a signal, output from the solid-state image detector when stimulating light incident on the image recording sheet is intercepted, is detected as a correction signal; and a non-image component, contained in a signal output from the solid-state image detector when scanning the image recording region with the stimulating light, is suppressed with the correction signal.
The expression xe2x80x9cwhen stimulating light incident on the image recording sheet is interceptedxe2x80x9d is intended to mean, for example, when stimulating light incident on the image recording sheet is intercepted by a shutter, etc.
In the first, second, and third methods of the present invention, it is preferable that the photoconductor of the solid-state image detector have amorphous selenium at its main component.
In accordance with the present invention, there is provided a first apparatus for carrying out the first method of the present invention. The first apparatus comprises scan means for scanning stimulating light in horizontal and vertical scanning directions on an image recording sheet having a stimulable phosphor layer which emits photostimulated luminescent light in a quantity corresponding to stored energy when irradiated with the stimulating light; a solid-state image detector with a photoconductor which generates electric charge when irradiated with the photostimulated luminescent light; acquisition means for obtaining an image signal in accordance with the quantity of the electric charge generated in the photoconductor; and suppression means for obtaining a signal, output from the solid-state image detector when scanning a non-image region other than an image recording region on the image recording sheet with the stimulating light, as a correction signal, and suppressing a non-image component, contained in a signal output from the solid-state image detector when scanning the image recording region with the stimulating light, with the correction signal.
In the first apparatus of the present invention, the correction signal obtained by the suppression means may be a signal obtained when scanning the vertical leading edge or trailing edge of the non-image region with the stimulating light. Also, the correction signal obtained by the suppression means may be a signal obtained when scanning the horizontal leading edge or trailing edge of the non-image region with the stimulating light.
In accordance with the present invention, there is provided a second apparatus for carrying out the second method of the present invention. The second apparatus comprises scan means for scanning stimulating light in horizontal and vertical scanning directions on an image recording sheet having a stimulable phosphor layer which emits photostimulated luminescent light in a quantity corresponding to stored energy when irradiated with the stimulating light; a solid-state image detector with a photoconductor which generates electric charge when irradiated with the photostimulated luminescent light; acquisition means for obtaining an image signal in accordance with the quantity of the electric charge generated in the photoconductor; control means for controlling an ON/OFF state of the stimulating light; and suppression means for obtaining a signal, output from the solid-state image detector when the stimulating light is in the OFF state, as a correction signal, and suppressing a non-image component, contained in a signal output from the solid-state image detector when scanning the image recording region with the stimulating light, with the correction signal.
The aforementioned xe2x80x9ccontrol meansxe2x80x9d refers, for example, to means for controlling the ON/OFF state of the stimulating light by sending an electrical signal directly to the stimulating-light source, and means for controlling the ON/OFF state of the stimulating light by controlling the ON/OFF state of the power source for the stimulating-light source.
In accordance with the present invention, there is provided a third apparatus for carrying out the third method of the present invention. The third apparatus comprises scan means for scanning stimulating light in horizontal and vertical scanning directions on an image recording sheet having a stimulable phosphor layer which emits photostimulated luminescent light by a quantity corresponding to stored energy when irradiated with the stimulating light; a solid-state image detector with a photoconductor which generates electric charge when irradiated with the photostimulated luminescent light; acquisition means for obtaining an image signal in accordance with the quantity of the electric charge generated in the photoconductor; interception means for intercepting stimulating light incident on the image recording sheet; and suppression means for obtaining a signal, output from the solid-state image detector when stimulating light incident on the image recording sheet is intercepted by the interception means, as a correction signal, and suppressing a non-image component, contained in a signal output from the solid-state image detector when scanning the image recording region with the stimulating light, with the correction signal.
In the aforementioned third apparatus, the xe2x80x9cinterception meansxe2x80x9d refers to a shutter, etc.
In the first, second, and third apparatuses of the present invention, it is preferable that the photoconductor of the solid-state image detector have amorphous selenium at its main component.
As described above, the first method and apparatus of the present invention use a signal, output from the solid-state image detector when scanning the non-image region with the stimulating light, in order to suppress a non-image component, contained in a signal output from the solid-state image detector when scanning the image recording region with the stimulating light. Thus, the first method and apparatus of the present invention are capable of reproducing a high-quality image, while removing a dark current component and a dark latent-image component which are steadily output from the detector.
The second method and apparatus of the present invention employ a signal, output from the solid-state image detector when the stimulating light is in an OFF state, in order to suppress the non-image component. Thus, the second method and apparatus of the present invention are capable of obtaining the same effect as the first method and apparatus of the present invention, with simpler construction and control.
The third method and apparatus of the present invention employ a signal, output from the solid-state image detector when stimulating light incident on the image recording sheet is intercepted, in order to suppress the non-image component. Therefore, the third method and apparatus of the present invention are also capable of obtaining the same effect as the second method and apparatus of the present invention.